1. Field of the Invention
The present invention relates to aramid honeycombs and a method for producing the same. More particularly, the invention relates to a honeycomb core using aramid honeycomb sheets as base materials, and a method therefor.
2. Description of the Prior Art
It has been known a honeycomb core structure comprising an assembly of a number of hollow, columnar honeycomb cells separated by cell walls adhered linearly to each other. Further, it has been also known to use aramid honeycomb sheets as base materials for the cell walls. Aramid honeycomb sheets comprising a nylon-type resin, particularly para-aramid honeycomb resin sheets, have flame-retardant, tough and other excellent properties required for a honeycomb core structure.
A honeycomb core structure comprising aramid honeycomb sheets used as the base materials for cell walls (hereinafter referred to as aramid honeycombs) is disclosed in Kokai (Jpn. unexamined patent publication) No. 4-226745. According to the publication, the aramid honeycomb sheets contain 0 to 50% by weight of a binder and 50 to 100% by weight of para-aramid fibers, wherein the para-aramid fibers represent 20% to 80% by volume of the total material.
The conventional aramid honeycombs, however, are defective in the following points.
First, the aramid sheets have a defective texture or structure. Sheets containing a large amount, i.e., 50% by weight or more, of para-aramid fibers are bulky and are inconvenient for handling. When the aramid paper sheets are produced, fibers are not uniformly stirred in a liquid such as water and, therefore, it is difficult to provide a papermaking treatment to the fibers to disperse the ingredients on a filter. Consequently, the produced aramid sheets are defective in that the para-aramid fibers are not uniformly dispersed and the sheet density is varied between portions of the sheets.
As a consequence, the sheets are wrinkled locally at portions differed in the number of para-aramid fibers, causing inconvenience in the operation of the production of the honeycombs. Further, a number of large pinholes are produced in sheets through which adhesives pass from one side surface to the other side. For these and other reasons, the production of the aramid honeycombs is often troubled.
Second, the paper sheet strength is defective. Since the sheets contain a large amount, i.e., 50% or more, of para-aramid fibers, Freeness value, i.e., a water-maintaining property and a water-filtering property, is unsatisfactory, resulting in the reduction in the bonding property of the binder mixed in a papermaking liquid, such as water. Since the binder is flowed off without being fixed to fibers and, thus, since the fibers are not bonded to each other, the paper strength of the aramid sheets decreases. Thus, when such aramid sheets are used as base materials, the produced honeycombs had an insufficient toughness.
Fibers have high restoring property. Therefore, thirdly, sheets become excessively thick when they are produced with a large amount, i.e., 50% or more, of para-aramid fibers since the fibers restore their volume after the sheets are produced and the sheets get thick, causing a reduction in sheet density. It was pointed out, therefore, that such thick sheets are inappropriate as base materials for honeycombs.
As has been mentioned above, the aramid honeycombs disclosed in said Kokai 4-226745 have problems caused by the sheets used as base materials. As a counter measure, meta-aramid pulps which act also as a binder were used and aramid sheets were provided with a calender treatment under a high. temperature and at a high pressure to be used as base materials for aramid honeycombs.
That is, meta-aramid pulps, which are excellent in a fixing property, have been used in addition to the main ingredient, i.e., 50% or more of para-aramid fibers, to overcome the problems of the lack in the fixing property and of the week paper strength. Further, the aramid sheets were, after provided with a paper-making treatment, calendered at a high temperature and under a high pressure to produce a thin film so as to overcome the third problems of being a thick sheet and of a reduction in density.
The meta-aramid pulp has a high softening point of over 200xc2x0 C. Therefore, para-aramid fibers and meta-aramid pulps are merely entangled with each other in the sheet produced by a normal method with a paper-making liquid such as water and dried at 150xc2x0 C. or lower.
That is, the meta-aramid pulps are not bonded in a liquid form to the para-aramid fibers, as is the case with a normal binder. Therefore, the sheets thus produced have not the strength sufficient for the production of aramid honeycombs.
To obtain the necessary strength, a high temperature and high pressure calendering treatment was needed according to the conventional processes. In fact, the aramid sheets after produced have been calendered under a high pressure of 29.4xc3x97104N/m(300 kg/cm) applied linearly and at a high temperature of around 300xc2x0 C. In this way, the meta-aramid pulps were softened, melted, fluidized and then hardened to act just as a normal binder to obtain a sheet strength necessary for the production of aramid honeycombs.
Second, aramid honeycombs were produced using the aramid sheets as base materials by a conventional enlarging process which comprises the steps of applying adhesives linearly to the sheets, piling the sheets such that each of the sheets are shifted by a half pitch of the linearly applied adhesives, applying pressure to the piled sheets under heating to bond the sheets to each other, and enlarging the sheets to the direction counter to the piling direction to obtain aramid honeycombs comprising cell walls of the aramid sheets. The aramid honeycombs thus obtained were provided with an after treatment so that the cell walls are coated by and impregnated with a reinforcing resin.
For the conventional aramid honeycombs and the method therefor, following problems have been pointed out.
First, no improvement has been provided for solving the problem of the defective texture or structure.
The para-aramid fibers are still not uniformly dispersed and so are unevenly present in the sheets. The meta-aramid pulps, fluidized by the high pressure and high temperature calendering treatment, are used only to fill in between para-aramid fibers. Consequently, the density in sheets is varied locally and is scattered. Thus, the heat shrinkage ratio is different between the portions containing much fibers and those containing less fibers, and the sheets at the cooling stage are not uniformly shrank under heat.
Therefore, the aramid sheets conventionally used are not smooth, and are liable to be wrinkled. Since the sheets are defective in preciseness, they are difficult in handling and are difficult to be piled precisely.
Further, there are cases where a number of large pinholes are formed in the aramid sheets. As the result, the adhesives applied linearly to the surface pass through the pinholes to the other surface under a high pressure and a high temperature, whereby each of the piled sheets is adhered to each other to form a block and the respective sheets are unable to be enlarged or expanded.
Second, a problem of the cost was pointed. For the conventional aramid sheets, expensive meta-aramid pulps are used and the honeycombs using the expensive aramid sheets are accordingly expensive.
Further, the conventional aramid sheets are provided with a calendering treatment under high temperature and high pressure. The cost for the treatment is added to the total production cost.
Third problem is related to the insufficient strength of the aramid honeycombs. The aramid sheets used as base materials for the honeycombs are calendered under a linear pressure of 29.4xc3x97104N/m(300 kg/cm) and at a temperature of around 300xc2x0 C., and no many pores are retained since the melted, fluidized and hardened meta-aramid pulp fill in pores. After the sheets are formed to honeycombs, a reinforcing resin is applied to cell walls to strengthen the honeycombs.
According to the conventional aramid honeycombs, the resin covers only the sheet surface and does not penetrate thereinto since no many pores remain in the sheets.
As the result, the aramid honeycombs are easily broken by an external crushing or shearing force. That is, when the external force is applied, the reinforcing resin layer is apt to be peeled off at the interface, i.e., at the outer surface of the aramid sheets which form the cell walls, and so the layer sometimes does not work effectively for reinforcement.
Since the reinforcing resin is apt to be peeled off, there is a problem in the honeycomb strength. To solve the problem, the cell walls should be denser, causing another problem of heavier aramid honeycombs.
The present invention has been made to solve the above-mentioned drawbacks related to the conventional arts. According to the present invention, there are provided aramid honeycombs produced using para-aramid sheets comprising para-aramid pulps and a binder. Optionally 40% or less of para-aramid fibers are contained to form 100% with the para-aramid pulps.
The para-aramid sheets are calendered by a linear pressure of 19.6xc3x97104N/m or more and a temperature of 150xc2x0 C. or higher such that the sheets have many pores with a porosity of 20% to 60% by volume. By an after treatment, a reinforcing resin is adhered to cell walls and the cell walls are impregnated with the resin.
The technical advantages obtained by the invention are that the texture or structure of the aramid sheets is improved whereby the honeycombs are produced smoothly, that the production cost is reduced and that the produced para-aramid honeycombs have an improved honeycomb strength.
According to a first form of the invention, there are provided aramid-honeycombs comprising an assembly of a number of hollow columnar cells separated by cell walls, wherein
a number of aramid sheets are used as the base materials for the cell walls, and said aramid sheets comprise para-aramid pulps and a binder and containing many pores after being provided with a calendering treatment, and
the cell walls formed by the aramid sheets are coated by and are impregnated with a reinforcing resin, and pores in the cell walls also are filled with the resin.
According to a second form of the invention, there are provided aramid honeycombs according to the first form, but wherein said aramid sheets further comprise para-aramid fibers.
According to a third form of the invention, there are provided aramid honeycombs according to the second form, but wherein the para-aramid fibers comprise staple fibers of a staple or flock shape, and are used in an amount of 40% by weight or less of the total amount (100%) of the para-aramid pulps and said para-aramid fibers, and wherein the binder is used in an amount of 5% to 20% by weight to the total amount (100%) of the para-aramid pulps and said para-aramid fibers, i.e., 5-20:100.
According to a fourth form of the invention, there are provided aramid honeycombs according to the first form, but wherein the pores are produced from the surface to the inside of the aramid sheets, and have a porosity of 20% to 60% by volume.
According to a fifth form of the invention, there are provided aramid honeycombs according to the fourth form, but wherein 50% by volume or more of the pores are filled with the reinforcing resin.
According to a sixth form of the invention, there is provided a method for producing aramid honeycombs, which comprises the steps of:
preparing aramid sheets comprising para-aramid pulps, para-aramid fibers and a binder,
calendering the aramid sheets such that many pores are produced and that the pores are adjusted to be retained,
applying adhesives linearly to the aramid sheets,.
piling the aramid sheets such that each of the sheets is shifted to the other by half a pitch of the lines of the applied adhesives,
applying pressure to the piled sheets under heating to bond the sheets with each other,
enlarging the sheets to the direction counter to the piling direction to obtain aramid honeycombs comprising an assembly of a number of cells separated by cell walls composed of the aramid sheets as the base materials, and
providing an after treatment of adhering a reinforcing resin to the cell walls and of impregnation so that the pores may be filled with the resin.
According to a seventh form of the invention, there is provided a method for producing aramid honeycombs according to the sixth form, but wherein the calendering step is carried out with a linear pressure of 19.6xc3x97104N/m or more and at a temperature of 150xc2x0 C. or higher.
Thus, according to the present invention, aramid honeycombs are produced from para-aramid sheets which comprise 60% to 100% by weight of para-aramid pulps, 40% to 0% by weight of para-aramid fibers, and 5 to 20% by weight to the total of the pulps and the fibers, i.e., 100: 5-20.
The aramid sheets are calendered with a linear pressure of 19.6xc3x97104N/m or more and at a temperature of 150xc2x0 C. or higher in such a manner that many pores are produced with a porosity of 20% to 60% by volume.
The cell walls, which produce the aramid honeycombs, are formed by applying adhesives linearly to aramid sheets, piling the sheets such that each of the sheets is shifted at a predetermined interval, applying heat and pressure to bond the sheets, and enlarging the sheets to the direction counter to the piling direction.
The aramid honeycombs thus formed are then provided with an after treatment by which cell walls are coated by and are impregnated with a reinforcing resin and the pores are filled in an amount of 50% or more with the resin.
The resin filling and penetrating into the inside of cell walls bonds strongly the para aramid pulps, para-aramid fibers and the binder constituting the cell walls. Further, the resin adhered to the surface of the cell walls is bonded three-dimensionally to the resin filling the inside of the cell walls.